I bought this battery pack thingy just an hour ago:
CLICKYIt comes with a screw driver and u can put ur old battery in it too. Together = 20 hours of gaming. Well that's what they say anyway... U could also buy some more Li-on batteries with it cuz it's really easy to change them when you put that thing on ur DS.
But maybe this is a cheaper option:
CLICKY!Needs 4 AAA batteries to play 9,5 hours without light and 7 with light.